A microwave oven cooks food by bombarding the food with electromagnetic waves which cause molecules in the food to vibrate billions of times per second. The heat is created when dipolar molecules (such as water) vibrate back and forth aligning themselves with the electric field or when the ions migrate in response to the electric field.
The vibrations cause heat by friction, although only at a depth of about 1 to 1.5 inches. Heat transfer properties of food continue the process of cooking by transmitting heat to areas of the cooking food that are relatively cool in comparison to the areas that have been heated by the electromagnetic waves.
Convenience of the microwave oven and reduced preparation time are key factors in the success of the microwave oven. Taste and quality of the food after being cooked in the microwave oven were at times lacking with early models because of inconsistent voltage management, inaccurately controlled magnetron tubes, and imperfect software control. Convenience was also lacking because as the demand for microwavable food increased so did the complexity of instructions for cooking that food. Imprecision of cooking instructions was fostered by, among other factors, the differing user interfaces. Other factors include operational characteristics of dissimilar and similar sized microwave ovens and allied microwave oven operational control and user interface disparities. Consumers want the convenience of microwave cooking but do not want to constantly refer back to a package to enter and re-enter multi-step instructions into a microwave oven to obtain cooked food, and still, after all their efforts receive sub-standard cooking results due to microwave oven operational and performance variances.
Because of more active lifestyles and less time spent in the kitchen, consumer demand for microwavable products is increasing along with the demand for a microwave oven that does not require a plurality of instructions to cook food, or different instructions for the same food item for different size and/or manufactured microwave ovens. Complicating the issue of product demand and usable microwave ovens is the wide variance in magnetron output power, performance variances, and user control interfaces now prevalent in the available universe of microwave ovens. A food product that may cook very well in a 1200 watt oven may take three times as long in an oven which can only provide 600 watts of power. Moreover, the user interface from microwave ovens of one manufacturer to another is often markedly different and non-intuitive.
Further complicating the issue of the wide variation in magnetron tube output power is the local utility (power company) that supplies power to the microwave oven of the user. Utility companies are often unable to balance adequately user demand for power with available power generation capability. The effects of power fluctuations on a microwave oven are numerous. In particular, the suggested cooking instructions for a particular food becomes meaningless. An example of this would be a power fluctuation of 6% by the public utility or power generation source for a brief period of time. The results of the degradation of power supplied to the microwave oven will be food that is undercooked. This may very well result in health hazards to the consumer of the food cooked in a microwave oven if bacteria are not killed by sufficient cooking. The sensitivity of output power to line voltage is a source of concern to the microwave oven food developer as well as the consumer. Measured power as a function of line voltage is shown in FIG. 18 for three commercially available microwave ovens. Note the variation of the 500 watt number two oven indicating a 6% change in line voltage. The output power of the magnetron tube of the microwave oven has decreased from 500 watts to 375 watts. Also, note the non-linear relationship between line voltage and power output of the magnetron tube of the microwave oven. This non-linear relationship will produce wide swings in output power due to rather small changes in line voltage (Microwave Cooking and Processing, Charles R. Buffler).
Microwave ovens presently in use employ various data entry mechanisms to input data into an oven control mechanism. These data entry mechanisms may be electrical and mechanical keyboards, card readers, light pens, wands, or the like. The control mechanism may be a computer or a microprocessor based controller. In general, the computer or controller has a basic input and output system (BIOS) associated with the input and output of data to and from the data entry mechanism. In such microwave ovens the user manually actuates the data entry mechanism to enter data relating to the type or mode of oven operation desired, i.e., bake, roast, re-heat, etc., as well as the length of the desired cooking time.
Present microprocessor-based controllers are capable of receiving a substantial amount of complex information from their associated data entry mechanism. This requires the oven user or process stream designer to manually enter a substantial amount of information generally in a multi-step series of data inputs on a keyboard. This information could be entered by a magnetic card containing all of the required input data, but this type of format does not allow flexibility in changing the cooking instructions. Alternately, user input could recall a stored recipe specific to a particular food item. Those familiar with the art can understand that an item-specific stored recipe system is static and inherently limited to the universe of food items known to its author at its moment of creation. Such a system is closed to food items or processes created subsequent to its moment of manufacture. Such a system is a stored recipe system specific and limited to a single host microwave oven or process stream.
In the manufacture of consumer appliances, such as microwave ovens, it is advantageous to assume that the overall control requirements are nearly the same from model to model. This is done to reduce the cost of manufacture of the microwave ovens and make the repair of the ovens more economical. The functions of the microwave oven such as "auto cook," "auto defrost" and a number of other cooking parameters associated with these functions vary from model to model, depending upon such factors as microwave cavity size, magnetron size, and other factors well known to practitioners in the art. Thus a controller may be required to operate correctly in different microwave oven chassis having different oven cooking cavities. Typical oven cavity size ranges from about 0.5 cubic feet to about 2.0 cubic feet. The ovens also may vary in their effective magnetron power output, one to another of the same model, and for a single oven from one use to another, depending on the electrical power supplied to it.
A well-known oven output power phenomenon concerning the mass of a specimen is documented in the IEC 705 publication. This publication defines a procedure for determining the output power of a microwave oven. Following the IEC 705 procedure a 1000 ml specimen of water is placed in a microwave oven. Power is applied to the specimen by the magnetron tube. The water boils at a specific power level in a given time period. The results of this test generated a classification of 800 watts for this particular microwave oven.
To further explain the phenomenon another test may be constructed following IEC 705 procedures. A specimen containing 250 ml of water is placed in the same microwave oven that was used to test the 1000 ml specimen and power is applied to the specimen. Performing the same calculations as before the microwave oven now appears to be a 660 watt oven. This particular phenomenon clearly asserts the specimen mass has a pronounced effect on determination of the power rating of the microwave oven.
Microwave power output can be controlled using two methods. The first is duty cycle control and the second is amplitude modulation. In duty cycle control, the average output can be adjusted by operating the magnetron at full rated power, while switching its current on and off for portions of a time interval. The percentage of time that the current is on during the time interval is referred to as the "duty cycle."
The duty cycle of the microwave oven is generally implemented by electromechanical relays in conjunction with the controls of the microwave oven. The relays provide economies of scale for a manufacturing effort but they do not adequately provide competent electrical current switching.
Magnetron power output is proportional to its cathode current. In amplitude modulation, the cathode current is adjusted to control the instantaneous magnetron output. The instantaneous magnetron current is controlled either by varying the level of high voltage to the magnetron or by changing the magnetic field intensity in the magnetron.
Attempts in the past have been made to monitor magnetron tube power and compensate for the fluctuations in power produced by the magnetron tube. It is well known in the art that power produced by a magnetron tube and delivered to a sample in a thermally cold oven is substantially different from that same magnetron tube in a thermally hot oven. A thermally cold oven is defined to be a microwave oven that has zero percent (0%) microwave emission for an extended period of time measured at ambient temperature. A thermal hot microwave oven is defined to an oven that has had one hundred percent (100%) microwave emission for an extended period of time. An example of thermal activity or lack thereof would be a microwave oven at 0% or 100% microwave emission that has thermally stabilized both within the magnetron and within the cooking cavity at room temperature. The period of stabilization may range up to several hours depending on numerous thermal conductive variables in place on or near the microwave oven that may produce thermal cooling thereby affecting the thermal stabilization of the microwave oven. In general and in normal households the microwave oven will thermal stabilize at 0% microwave emission in about one to three hours. It is also well known in the art that when the operating temperature of a magnetron tube increases the power produced decreases. The operating temperature of the magnetron tube will increase due to normal operation. The heat produced by the specimen contained within the microwave oven having work performed thereon will also increase the temperature of the magnetron tube and the cooking cavity. The specimen does not consume 100% of the power generated by the magnetron tube; therefore, some of that power will be radiated outwardly from the specimen in the form of heat. Given the close proximity of the magnetron tube to the specimen the magnetron tube operating temperature will undoubtedly increase.
Monitoring the output of the microwave oven and then increasing input power to raise the power output of the magnetron tube is a self-defeating effort. As more power is supplied to the magnetron tube the power output of the magnetron tube increases, but the efficiency of the magnetron tube decreases, thereby increasing the operating temperature. This means the input power should be increased to compensate for the decrease in output power. This process will continue until a maximum input power is achieved thereby saturating the magnetron tube and further decreasing efficiency of the magnetron tube.
Another method of monitoring power output of the magnetron tube is to compare the monitored value of power to the power being delivered to the microwave oven by the power utility company. If these values do not compare after subtracting known losses, a compensation factor extracted from a lookup table has to be determined. This determined correction factor is mechanically or electronically applied to the magnetron tube. Applying this factor in this manner will increase or decrease the amount of power delivered to the magnetron tube. This is a self-defeating effort. If the magnetron tube power is too high the magnetron tube operating temperature will increase causing a decrease in efficiency, as discussed above. This results in a new compensation factor being applied to the magnetron tube power level. This cycle of applying correction factors and adjusting power levels will continue and the result of this effort will not correct the work performed on the specimen disposed within the microwave oven.
It is a well-known principle of physics that when a force does work on an object it must increase the energy of that object by a like amount (or decrease if the work is negative). When an object loses energy of any form, it must experience a like increase in energy of some other form, or it must do a like amount of work. Power discussed herein is the time rate of doing work. Power is expressed as an equation: Work=Power.times.Time.
Microwave ovens having compatible hardware can interact and share data. It has been possible in the past to exchange software between identical types of machines. To the contrary, most interactions between incompatible machines still involve little more than simple transfer of data files or the like. Software applications written for one microwave oven manufacturer or for one specific type of operating environment, however, cannot normally be ported or "transferred" to a system having different physical characteristics without being entirely rewritten. While much progress has made in developing techniques for exchanging data between incompatible machines, it has not been possible to exchange software application programs between different microwave ovens.
Data presented in the form of recipe instructions that offer static cooking conditions differ on characteristics of the material to be cooked. The material inherently varies in dielectric property, relative dielectric constant, and loss factor. These properties govern both the heating rate and uniformity, the latter being influenced by the depth of penetration of the microwave energy. Accordingly, conventional fixed cooking program functions do not allow the entry of data concerning the conditions of the material to be cooked into memory of the computer or controller of a microwave oven. As a result two materials would be cooked under the same cooking conditions in spite of having different material characteristics and cooking profiles. This causes an undesirable cooking operation.
It would be desirable to have a microwave oven or process control system that could accept pre-defined user entered programming information that could be interpreted and scaled to varying magnetron performance or process performance level(s) and power level duration(s) specific to a particular host unit. As the result of a single user entered pre-defined code, the final end result of a process performed for a particular item would be independent of and produce identical results upon the item regardless of the functional operating characteristics of any particular host microwave oven or process stream into which the user entered predefined code is input.